In the manufacture of containers for packaging products such as beer or beverages, numerous steps are necessary to produce the finished container. The most common type of container for beer and beverages in existence today is what is know as a two-piece container wherein a blank is initially formed into a cup and then is reformed through a drawing and ironing process into a thinned sidewall having an integral end wall which is normally domed inwardly. The drawn and ironed container is then trimmed to a predetermined height, is cleaned in a can washer and a label is then applied. Next, a coating is applied to the inside surface, and finally it is given a reduced neck.
One of the essential steps in the can making process is the internal coating of the container which is crucial, particularly when a black plate material or tin plate material is utilized for the stock material. It is absolutely essential that the integrity of the internal coating be maintained to insure that no uncoated surface remains exposed which can be subjected to a corrosion process, particularly when carbonated beverages are stored therein, which may render the product unacceptable.
Numerous proposals have been made for automatically inspecting the integrity of a coating on the inner surface of the container, but these have been unsatisfactory and have never been commercially acceptable. Thus, most of the inspection still occurs utilizing the visual method or filling the container with a liquid material and taking a reading to determine the amount of iron, if any, present after a predetermined time span.